gpio: davinci: Allocate the correct amount of memory for controller

Previously we created a controller structure per bank of GPIO pins. This
has since been changed to one per controller, but the allocation size
was not changed. Fix this here.

This also leaves the variable 'nbank' unused, instead of removing it,
move it down and use it to clean up a loop. For loops with multiple
initializers and/or iteration expressions, especially ones that don't
use those loop counters are quite hard to follow, fix this.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Acked-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>